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What is ORP, how is it
measured, and what is ORP, Eh and SHE? |
EnviroEquip
Product
News,
March 2003, updated Jul 2007
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What is ORP? Oxidation Reduction
Potential (ORP, or Redox) is a measure of a water system’s capacity to
either release or gain electrons in chemical reactions. The process of
oxidation involves losing electrons while reduction involves gaining
electrons.
Oxidation and reduction (redox) reactions control the behaviour of many
chemical constituents in drinking water, wastewater, and aquatic
environments. The reactivity and solubility of critical elements in living
systems is strongly dependent on redox conditions. ORP values are used
much like pH values to determine water quality. While pH values
characterise the relative state of a system for receiving or donating
hydrogen ions (acting as a base or an acid), ORP values characterise the
relative state of a system for gaining or losing electrons. ORP values are
affected by all oxidising and reducing agents, not just acids and bases.
Why Measure ORP?
The effect that potable water has on plumbing is directly related to
its ORP value. Unfavourable values can cause excessive corrosion, leading
to expensive repairs. ORP is one parameter that can be monitored during
the disinfecting process for drinking water, swimming pool water, and spa
water.
The life expectancy of bacteria in water is related to ORP. In fact,
studies have shown that the life span of bacteria in water is more
dependent on the ORP value than on the chlorine concentration. For
swimming pools at a normal pH value between 7.2 and 7.6, the ORP value
must be kept above 700 mV to kill unwanted organisms. ORP data unless
specific information about the site is known.
Hypochlorite or other oxidising agents must be added when the ORP falls
below 700 mV. In contrast, natural waters need a much lower ORP value in
order to support life. Generally ORP values above 400 mV are harmful to
aquatic life. Ideally the ORP value in salt water aquariums should be kept
between 350 and 390 mV. ORP levels below 300 mV are to be avoided. An
oxidising environment is needed to convert any ammonia (NH3) to nitrites
(NO2–) and nitrates (NO3–). Ammonia levels as low as 0.002 mg/l can be
harmful to some fish species.
Measuring ORP in Groundwater, Surface Water and Waste Water
The determination of ORP is particularly worthwhile in water that
contains a relatively high concentration of a redox-active species, e.g.,
the salts of many metals (Fe 2+
, Fe 3+
) and strong oxidising (chlorine) and
reducing (sulfite ion) agents. Thus, ORP can sometimes be utilised to
track the metallic pollution of ground or surface water, or to determine
the chlorine content of wastewater effluent. However, ORP is a
non-specific measurement—that is, the measured potential is reflective of
a combination of the effects of all the dissolved species in the medium.
Because of this factor, the measurement of ORP in relatively clean
environmental water (ground, surface, estuarine, and marine) has only
limited utility unless a predominant redox-active species is known to be
present. Care is required not to "over-interpret" ORP data unless specific
information about the site is known.
The ORP Field Sensor
The operation of the ORP sensor is very similar to that of the pH
sensor. A two-electrode system is used to make a potentiometric
measurement. The ORP electrode serves as an electron donor or electron
acceptor depending upon the test solution. A reference electrode is used
to supply a constant stable output for comparison. Electrical contact is
made with the solution using a saturated potassium chloride (KCl)
solution. The electrode behaviour is described by the Nernst equation:
E m
= E o
- (RT/nF) ln {[ox] / [red]}
where
E m
is the potential from the ORP electrode,
E o
is related to the potential of the reference electrode,
R is the Gas Law constant,
F is Faraday’s constant,
T is the temperature in Kelvin,
n is the number of electrons,
[ox] is the oxidant concentration in moles/L, and
[red] is the reductant concentration in moles/L.
Most natural waters contain many species that are involved in the redox
process so that it is not possible to calculate the ORP using the Nernst
equation. All redox species do however reach equilibrium. A Standard
solution of known redox potential for a particular ORP electrode is used
to calibrate the ORP sensor. The ORP sensor then gives a calibrated
response in mV when placed in a sample.
| Typical ORP values |
mV |
| Properly chlorinated swimming pool
|
> 700 |
| Salt water aquarium |
~ 350 |
| Harmful to aquatic life |
> 400 |
References
- In-Situ Inc, MP TROLL 9000 Operations Manual 0042400 rev. 000
03/02, 2002.
- Eaton, A.D., L.S. Clesceri, and A.E. Greenberg, eds., Standard
Methods for the Examination of Water and Wastewater, 19th
edition, Washington, D.C.: American Public Health Association, American
Water Works Association, and Water Environment Federation, 1995. Section
2580, Oxidation-Reduction Potential.
- Ionode Pty Ltd Intermediate Junction pH/Redox/Reference
Instruction Manual IJ44/IJ46/tJ64/IJ14/IJ16
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Measuring water quality with the TROLL 9000
Eh and SHE
ORP is measured in units of millivolts (mV), or Eh (1 Eh = 1mV). The
standard hydrogen electrode (SHE) is the reference from which all
standard redox potentials are determined, and has been assigned an
arbitrary half cell potential of 0.0mV. However the SHE is fragile
and impractical for routine laboratory and field use. Therefore Ag/AgCl
and saturated calomel reference voltages are commonly used. The
voltages of the different reference electrodes can be interrelated
with respect to the SHE reference electrode, as per below:
-
TPS 90FLMV
add 199 to your reading
- In-Situ
MP TROLL
9500: add 200 to your reading
- Hanna ORP and
combination meters - add 204mV to your reading
So if using the
90FLMV, and your ORP reading is -150mV, this becomes 49mV in terms
of the SHE. |
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